Negative resistance device for generating oscillations or reducing damping



Sept- 1946- P. K. CHATTERJEA ETAL 2,406,804

\ NEGATIVE RESISTANCE DEVICE FOR GENERATING OSCILLATIONS OR REDUCING DAMPING Filed Nov. 6, 1943 VOLTAGE CURRENT,

Patented Sept. 3, 1946 N EGATIV E1 RESISTANCE DEVICE. FDR' GEN ERATING OSCIL'IJATIONS" OR REDUCING DAMPING Prafulla Kuniar Chatterjea and Charles Thomas Scully, London, England, assignors, bymesnc assignments, to International Standard Electric Corporation; New York, N. Y.', a corporation oi- Delaware Application November 6, 1943, Serial No. 509,188 in Great Britain .ianuary S, 1943..

1 Claim: 1.;

The "present invention relates to clectricresonan't circuits particularly those used in oscillation generators and employs the special prop ei'ties of thextherinally sensitive'resistance elements known as thermistors.

Thermistors are thermosensitive resistance ele mentshavinga temperature coefficient of resistancevvhich may be either positive or negative-and which is moreover many times the corresponding coefficient for a pure metal such as copper; This propertyrenders thermistors particularly suitable for a variety of special applications in electric circuits.

Various differentmaterials are available for the resistance element of a thermistor, these various materials having different properties in other respects; as: one example, a-resistance material having a high negative temperature coefficient of resistance comprises a mixture of manganese oxide, and nickel oxide'w-ith or'without'the-addition of certain othermetallic oxides, the mixture being suitably heat treated.

Thermistors have been employedin two different forms: (a) knownas a -directly heated thermistor and comprising a resistance element of the thermally sensitive resistance material provided with'suitable lead-out conductors or terminals, and (b) known as'an' indir'ectly heated thermistor comprising 'tlie'ele'ment (a) provided in addition with a heating coil electrically-m A directly 1 heated thermistor is primarily intended to b'e controlled sulated from the element.

by the currentwhich flows through it and which variesthe temperature and also the resistanceaccordingly. Such thermistor wil l 'also b'e affected bythe temperature of its surroundings-and maytherefore be used for thermostatic control andlike purposes-With or Without direct'h'eating by the current flowing through it. An indirectly heated thermistor is chiefly designed to be heated bya controlling current which flowsthrough the heating coil and'which will usually, but not necessarily, be different from the current which flows through the resistance element, but this type of thermistor may also be subjected to either or both of the types of control applicable to a directly heated thermistor.

More detailed information on the properties of thermistors will be found in an article by G. L. Pearson in the Bell Laboratories Record Dec. 1940, page 106.

In the present invention, thermistors having a negative temperature coeiiicient of resistance are employed, since under appropriate conditions they may behave like negative resistances and thereforemay. be usedto generatexoscillations in a resonant circuit in; a: particularly simple way;

By suitablexacljustment of the'constants' of the. circuit; oscillations may just :be' prevented. from occurring;- and: in"v that casea resonant circuit, which may have a very low resonance frequency and a very highvalueof Q", .may be easily "ob tained foruse infilter and. other like circuits.

According. toone aspect'of the invention; therefore, theretis provided 'an electric resonant cir.-' cuit comprising an'inductance, a thermistor having .a: negative temperature coefficient of resist;

ance, and a source of direct current; coupled;

together in such a manner that the. ratio of "re-1 actanceito resistance of the inductance. is efliec'- tively raised to a; predetermined value.

Accordingto another aspect; thezinvention'provides anelectric: resonant circuit comprisingan' inductance at least part of which is connected" in series with the resistance element of fa thermistor having a negative temperature coeificient of resistance, to'avsource of direct current, theth'ere mistorvcircuit resistancebeing adjustedso that the voltage-current characteristic curve of; the thermistor circuit'h'as a portion; whose slope is slightlypositive,. and. so "that the current through the-resistance element is represented-byapoint on the said portion of slightly positive slope;

Accordingto the invention, also; the thermistoreircuit resistancein either of the abovemem. tioned aspects. may be alternatively adjusted =50 that oscillations take place in thereson-antcircuit.

The invention will be described with reference to the accompanying drawing in which:

Fig; 1 shows'an exampleof one circuit-according to" the invention; and

Fig; 2fshows characteristic curves used to -ex-- plain the operation'of'Fig. 1.

It will be understood that any'resistances other:

than thermistorsemp-loyed in'this inventionare of the ordinary kind in which the resistance value is substantially:-independent of the current.

Fig. 1 shows an example of a circuit according to the invention. An indirectly heated thermistor T having a negative temperature coefficient of resistance has its resistance element R connected in series with part of an inductance L and a tapped battery or other direct current source E. The heating coil 1' of the thermistor is connected to the battery in series with an adjustable resistance R1. A parallel resonant circuit is produced by connecting a condenser C across the inductance L.

In Fig. 2 are shown characteristic curves for the circuit of Fig. 1. The abscissae represent the currents and the ordinates the corresponding voltages.

Curve A shows the curve for the thermistor T alone, with no current in the heating coil. As the current through the resistance element R is increased, the voltage across R first increases to a maximum and then decreases continuously. The part of the curve beyond the maximum has a negative slope, and this means that the effective resistance to current changes is negative.

The straight line B represents the characteristic for the resistance of that part of the inductance L which is in series with the thermistor (together with any other resistance there may be in the circuit) and this resistance is assumed to be substantially constant. Curve C has been ob tained by adding the ordinates of curves A and B, and represents the resistance characteristic of the whole of the circuit. It will be seen that curve C is horizontal in the neighbourhood of the point P signifying that the effective resistance to changing currents is zero. According to known principles, the circuit will oscillate in the neighbourhood of the point P at a frequency demay also be controlled by including a suitable resistance (not shown) in series with the thermistor element R. This will change the slope of the line B in Fig. 2.

The oscillation output may be taken from the tuned circuit L, C in any suitable way (not shown). Although adjustable connections to the battery and to the inductance L are indicated for convenience, these could of course be fixed; and if adjustment of the characteristics of the thermistor are not required, one of the directly heated type could be used, the resistance R1 and its connections being omitted.

The tuned circuit could furthermore be coupled to the thermistor circuit by means of a separate winding (not shown) coupled to the inductance L, and connected in series with the thermistor and source E, and various other modifications will occur to those skilled in the art.

It should be added that owing to the fact that the response of the thermistor to changes in the current is not instantaneous, the frequencies to which the circuit is applicable will generally be low. Thermistors having very rapid response are however known, and frequencies of several hundred periods per second may be possible.

The circuit of Fig. 1 is particularly simple since it does not contain any valves which would re quire a high tension or cathode heating source. It would be useful, for example, as a low frequency generator; as a vibrator for power packs, or to generate ringing frequencies for telephone circuits.

Referring again to Fig. 2, if the resistance connected in series with the resistance element R be increased, the slope of the curve B will be also increased and the horizontal part of the curve C will move to the left until it reaches the voltage maXimum of the curve. At, or a little after, this stage, the curve C will be found to have no horizontal portion, and no portion of negative slope, but there will be a short region in the neighbourhood of the maximum of the curve where the slope is only very slightly positive. Under these conditions oscillations will not ocour, but the ratio Q of reactance to resistance of the inductance L may effectively be very high, and can be given any desired value by suitably adjusting the circuit. In this way a resonant circuit L, C tuned to a very low frequency, and having a very high value of Q, is obtained in a very simple way; this is well known to be dinicult at low frequencies, requiring very bulky coils or complicated amplifying arrangements by known methods.

It may be added that another method of obtaining the desired slightly positive slope is to adjust the resistance R1 in Fig. 1 instead of adjusting the resistance in series with R. By increasing the current through the heating coil 1- the horizontal portion of the curve C will also move to the left until it disappears in the neighbourhood of the maximum of the curve. It is further to be noted that in Fig. 1, the current for the heating coil r could, if desired, be obtained from a direct or alternating current source (not shown) entirely separate from the direct current source E.

What is claimed is:

Electrical circuit including, connected in series, an indirectly heated, negative characteristic thermistor, a source of direct current, and at least part of an inductance, also including a capacity shunting said inductance and forming therewith a resonant circuit, said series connected circuit also including a variable resistance connected in series with the heater of said thermistor, whereby the characteristic voltage-current curve of said thermistor may be altered in shape and position, so that a given current flow therethrough may be made to lie at a predetermined point of said curve, said point being located at a portion of said curve having suflicient negative slope to cause said resonant circuit to oscillate.

PRAFULLA KUMAR CHATTERJE'A. CHARLES THOMAS SCULLY. 

